Method for ink jetting opaque ink composition

ABSTRACT

A method of inkjet printing includes ink jetting an aqueous inkjet ink composition to form a white printed image on a substrate. The aqueous inkjet ink composition comprises particles of titanium dioxide that are present in an amount of at least 4 to 15 weight % and the particles have a 95 th  percentile particle size of less than 200 nm and a 50 th  percentile particle size of less than 130 nm. The titanium dioxide particles are dispersed within the aqueous inkjet ink composition using a dispersing polymer that is a styrene-(meth)acrylic acid polymer, styrene-maleic anhydride polymer, or styrene-maleic acid polymer and the weight ratio of the titanium dioxide particles to the dispersing polymer is from 19:1 to and including 2:1.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 62/259,234, filed Nov. 24, 2015, which is incorporatedherein by reference in its entirety.

Reference is made of the following copending and commonly assignedpatent applications, the disclosures of all of which are incorporatedherein by reference:

U.S. Ser. No. 15/218,132, filed on Jul. 25, 2016, by Bauer and Jones andentitled “Providing Opaque Ink Jetted Image” that has the benefit ofpriority of U.S. Ser. No. 62/259,222 filed Nov. 24, 2015;

U.S. Ser. No. 15/218,144, filed on Jul. 25, 2016, by Bauer, Bennett, andJones and entitled “Pigment Dispersions and Inkjet Ink Compositions”that has the benefit of priority of U.S. Ser. No. 62/259,226 filed Nov.24, 2015; and

U.S. Ser. No. 15/218,172, filed on Jul. 25, 2016, by Bauer, Bennett, andJones and entitled “Aqueous Colorant Dispersions and InkjetCompositions” that has the benefit of priority of U.S. Ser. No.62/259,238 filed Nov. 24, 2015.

FIELD OF THE INVENTION

This invention relates to aqueous dispersions including colorantparticles such as titanium dioxide particles, aqueous inkjet inkcompositions, and methods for forming pigmented images such as opaque(white) images on a substrate. The aqueous dispersions and aqueousinkjet ink compositions can include unique dispersing polymersespecially when titanium dioxide particles are used as the pigmentcolorant.

BACKGROUND OF THE INVENTION

Inkjet printing systems are generally of two types: drop on demand (DOD)printing systems and continuous inkjet (CIJ) printing systems. Inkjetprinting is a standard method for printing a colored or pigmented imageonto a substrate wherein a stream of ink droplets containing pigmentparticles are directed from a printing device to a surface of a suitablereceiver element or substrate. The direction of the stream of dropletsis controlled electronically so as to cause printing the drops in adesired image or information on the substrate surface without requiringcontact between the printing device and the surface to which the ink isapplied. Objects comprising substrates to which inkjet printing is wellsuited include but are not limited to, containers for consumer products,currency, draft checks, envelopes, letterhead, documents of varioustypes, identification cards, lottery tickets, bank cards, identificationstrips, labels, brochures, signage, and other well-known materials.

Drop-on-demand printing systems are widely used in home or consumerinkjet printers and slower consumer printers, both of which have beenavailable for several decades. As the name implies, this type of inkjetprinting uses a print head that ejects drops of ink only when signaledto do so by a digital controller.

CIJ printing systems generally comprise two main components, a fluidsystem (including an ink reservoir) and one or more print heads. Ink canbe pumped through a supply line from the ink reservoir to a manifoldthat distributes the ink to a plurality of orifices, typically arrangedin linear array(s), under sufficient pressure to cause ink streams toissue from the orifices of the print head(s). Stimulations can beapplied to the print head(s) to cause those ink streams to form streamsof uniformly sized and spaced drops that are deflected in a suitablemanner into printing or non-printing paths. Non-printing drops can bereturned to the ink reservoir using a drop catcher and a return line.Thus, in contrast to DOD printing systems, CIJ printing systems involvethe use of a continuous stream of ink drops that are separated todiscriminate between spaced printing drops and non-printing drops. Forexample, this discrimination can be accomplished by air deflection or byelectrostatically charging the drops and passing the charged dropsthrough an electric field. Charged drops are deflected by a charge fieldand can be caught and returned to the reservoir of ink. Uncharged dropsare printed onto a substrate or receiver material.

In general, pigment-based inks can comprise a wide variety of organic orinorganic pigments that can be chosen depending upon the specificapplication and performance requirements for the printing system anddesired printing results (for example, desired hue). For example, suchorganic or inorganic pigments can include but are not limited to,titanium dioxide or other white pigments, carbon black or other blackpigments, red pigments, green pigments, blue pigments, orange pigments,violet pigments, magenta pigments, yellow pigments, and cyan pigments.Iridescent and metallic pigments can also be used for special opticaleffects.

White inks can be prepared using high refractive index particles ofinorganic materials such as metal oxides including titanium dioxide.Such particles are most efficient at scattering actinic light withparticle sizes of from 200 nm to 1000 nm. However, because of theirdensity, such particles in this size range or larger do not form stableaqueous dispersions and will readily settle out even when a typicaldispersant is included in the aqueous dispersions or formulations.Aqueous white inks are described in U.S. Patent Application Publication2014/0292902 (Kagata et al.) in which metal oxides are mixed with binderresins such as polyester resins, fluorene resins, or styrene-acrylicresins, organic solvents (polyols or pyrrolidones), and surfactants. Themetal oxide particles are defined by using a specified relationship of astructural factor, porosity, particle diameter, and specific gravitywith the average particle size being greater than 150 nm and less than10,000 nm, and more preferably in the range of from 300 nm to 600 nm.

Other white inkjet ink compositions are described in U.S. Pat. No.7,850,774 (Oriakhi) in which the white pigment dispersions display abimodal distribution wherein at least 5 weight % of the particles havean average size equal to or greater than 100 nm and at least 25 weight %of the particles have an average size of equal to or less than 50 nm.

Still another aqueous inkjet ink composition is described in U.S. PatentApplication Publication 2014/0288208 (Sasada et al.), which compositioncomprises particles of titanium dioxide having an average primaryparticle diameter of 200 nm or more, a water-soluble resin, andself-dispersing resin particles having an average particle diameter of40 nm or less.

U.S. Pat. No. Application Publication 2014/0123874 (Kabalnov et al.)describes a white inkjet ink having high index refraction indexparticles having a diameter of less than 100 nm and low refraction indexparticles such as emulsion particles having a diameter of greater than100 nm.

U.S. Pat. No. 7,592,378 (Lin et al.) describes aqueous titanium dioxideslurries for inkjet processes in which the titanium dioxide pigment isdispersed using a combination of a graft copolymer and a blockcopolymer. The titanium dioxide particles are considered to have a morepreferred particle size of from 100 nm to 500 nm.

A mixture of a styrene acrylic resin and a urethane resin is used as afixing resin in white compositions according to U.S. Pat. No. 8,697,773(Okuda et al.). In these compositions, the more preferred particle sizeis 200 nm to 400 nm.

To obtain stable dispersions of inorganic pigments such as titaniumdioxide in the presence of dispersants, the pigment particle size shouldbe below 200 nm, and even less than 100 nm. However, at a particle sizethat is less than ½ the wavelength of actinic light, such inorganicpigment particles do not effectively scatter actinic light to create adry “white” coating of desired opacity.

Thus, there is a need to create stable (well dispersed) aqueousdispersions and aqueous inkjet ink compositions that comprise titaniumdioxide or other white pigments or even other colorants wherein theparticle size is generally below 200 nm for effective stability in theaqueous dispersions and inks and for effective actinic light scatteringwhen the particles aggregate into larger light scattering centers upondrying on a surface, thereby providing desired opaque images.

SUMMARY OF THE INVENTION

The present invention provides a method of inkjet printing, comprising:

providing a substrate, and

ink jetting an aqueous inkjet ink composition to form a white printedimage on the substrate,

wherein the aqueous inkjet ink composition comprises particles oftitanium dioxide that are present in an amount of at least 4 weight %and up to and including 15 weight %, and the titanium dioxide particleshave a 95^(th) percentile particle size of less than 200 nm and a50^(th) percentile particle size of less than 130 nm, and

the titanium dioxide particles are dispersed within the aqueous inkjetink composition with a dispersing polymer that is astyrene-(meth)acrylic acid polymer, styrene-maleic anhydride polymer, orstyrene-maleic acid polymer, wherein such dispersing polymer comprisesat least 25 mol % of styrene recurring units and at least 20 mol % ofrecurring units derived from (meth)acrylic acid, maleic anhydride ormaleic acid, respectively; and the weight ratio of the titanium dioxideparticles to the dispersing polymer is from 19:1 to and including 2:1.

In addition, the present invention provides a method of inkjet printing,comprising:

-   -   providing a substrate,    -   ink jetting an aqueous inkjet ink composition from a main fluid        supply as a continuous stream that is broken into both printing        drops and non-printing drops, forming a white printed image on        the substrate with the printing drops, and    -   collecting and returning the non-printing drops to the main        fluid supply,

wherein:

the aqueous inkjet ink composition comprises particles of titaniumdioxide that are present in an amount of at least 4 weight % and up toand including 15 weight %, and the titanium dioxide particles have a95^(th) percentile particle size of less than 200 nm and a 50^(th)percentile particle size of less than 130 nm, and

the titanium dioxide particles are dispersed within the aqueous inkjetink composition with a dispersing polymer that is astyrene-(meth)acrylic acid polymer, styrene-maleic anhydride polymer, orstyrene-maleic acid polymer, wherein such dispersing polymer comprisesat least 25 mol % of styrene recurring units and at least 20 mol % ofrecurring units derived from (meth)acrylic acid, maleic anhydride ormaleic acid, respectively; and the weight ratio of the titanium dioxideparticles to the dispersing polymer is from 19:1 to and including 2:1.

The present invention provides improved aqueous dispersions and aqueousinkjet ink compositions that can be used to provide dry opaque (“white”)or other dry colored pigment images on a surface (including speciallytreated substrates) using various inkjet processes including continuousinkjet processes. The particle size of the pigment colorants such astitanium dioxide particles is carefully controlled for desireddispersibility, ink jetting properties, and then successfulagglomeration of ink jetted particles on a substrate to exhibitsuccessful actinic light scattering (high refractive index) especiallywhen opaque images are desired. In some embodiments, titanium dioxideparticles or other pigment colorant particles are dispersed and inkjetted using dispersing polymers as described below.

DETAILED DESCRIPTION OF THE INVENTION

The following discussion is directed to various embodiments of thepresent invention and while some embodiments can be desirable forspecific uses, the disclosed embodiments should not be interpreted orotherwise considered to limit the scope of the present invention, asclaimed below. In addition, one skilled in the art will understand thatthe following disclosure has broader application than is explicitlydescribed in the discussion of any particular embodiment.

Definitions

As used herein to define various components of the various aqueousdispersions or aqueous inkjet ink compositions (whether containingtitanium dioxide or other pigments), unless otherwise indicated, thesingular forms “a,” “an,” and “the” are intended to include one or moreof the components (that is, including plurality referents).

Each term that is not explicitly defined in the present application isto be understood to have a meaning that is commonly accepted by thoseskilled in the art. If the construction of a term would render itmeaningless or essentially meaningless in its context, the termdefinition should be taken from a standard dictionary.

The use of numerical values in the various ranges specified herein,unless otherwise expressly indicated otherwise, are considered to beapproximations as though the minimum and maximum values within thestated ranges were both preceded by the word “about.” In this manner,slight variations above and below the stated ranges can be used toachieve substantially the same results as the values within the ranges.In addition, the disclosure of these ranges is intended as a continuousrange including every value between the minimum and maximum values.

As used herein, the parameter “acid number” (also known as acid value)is defined as the milligrams (mg) of potassium hydroxide required toneutralize 1 g of the described acidic polymer.

Unless otherwise indicated herein, the term “white” as applied to dryimages on a substrate is described by the image “whiteness index” thatcan be determined using the known standard ASTM E313-10.

Unless otherwise specified, the term “pigment colorant” includes any anaqueous-insoluble organic or inorganic material that can provide adesired color, hue, or other optical effect for example in an ink jettedimage. Such pigment colorants include metal oxides such as titaniumdioxide that can be used to provide aqueous dispersions and aqueousinkjet ink compositions. For such aqueous dispersions and aqueous inkjetink compositions when dried on a substrate, the opacity and hiding powerare dependent on the ability of such particles to scatter actinic lightacross the visible spectrum (generally wavelengths of at least 400 nmand up to and including 750 nm). The scattering intensity of actiniclight is known to be a strong function of the refractive indexdifference between the dried particles and the material surrounding theparticles, and the particle size of the pigment colorant particles. Asthis refractive index difference is increased, the greater the actiniclight scattering effect.

As used herein, the parameter “opacity” can be measured using an X-ritedensitometer. The density of an applied image according to thisinvention can be measured over the white and black calibration patchesprovided with the densitometer. The % opacity was calculated accordingto the following formula:

${\%\mspace{14mu}{Opacity}} = {100*\left( {1 - \frac{D_{{ink},{black}} - D_{{ink},{white}}}{D_{{support},{black}} - D_{{support},{white}}}} \right)}$wherein:

-   -   D_(ink,black)=density of the ink sample applied over the black        patch;    -   D_(ink,white)=density of the ink sample applied over the white        patch;    -   D_(support,black)=density of the support applied over the black        patch; and    -   D_(support,white)=density of the support applied over the white        patch.

The term “aqueous” in aqueous inkjet ink compositions as used in thepresent invention means that the water content is greater than 50 weight% based on the total aqueous inkjet ink composition weight. Water is thepredominant solvent (that is, it is more than 50 weight % of allsolvents).

Particle size for the various pigment colorants including titaniumdioxide refers to the approximate diameter of a generally sphericalpigment particle or to the approximate largest characteristic dimensionof a non-spherical particle. The 95^(th) percentile particle size refersto the classified particle size distribution such that 95% of the volumeof pigment colorant particles is provided by particles having diameterssmaller than the indicated diameter. Similarly, the 50^(th) percentileparticle size refers to the classified particle size distribution suchthat 50% of the volume of pigment colorant particles is provided byparticles having diameters smaller than the indicated diameter. The20^(th) percentile particle size refers to the classified particle sizedistribution such that 20% of the volume of pigment colorant particlesis provided by particles having diameters smaller than the indicateddiameter. Such particle size (or particle volume) evaluation can be madeusing a Nanotrac 150 ultrafine particle analyzer (Microtrac).

Titanium dioxide used in some embodiments of this invention is alsoknown as titania or titanium dioxide (IV) and can be any of a rutile,anatase crystalline, or brookite type, or it can be a mixture of two ormore of such types. Suitable commercial titanium dioxides are describedbelow in the Examples.

The pigment colorants used in the various embodiments of the presentinvention are generally not self-dispersing meaning that the pigmentcolorant particles are not purposely surface treated to providedispersibility in aqueous formulations, but some surface treatment canbe present as provided by a commercial manufacturer. Thus, such pigmentcolorants require the presence of one or more pigment dispersants suchas dispersing polymers described below to keep them suspended in anaqueous medium.

The terms “water-soluble” and “aqueous-soluble” mean that 2% by weightor more of a described material can be dissolved in distilled water at25° C., or particularly at least 5% by weight of such material can be sodissolved and the resulting solution is generally homogeneous andvisually clear.

For clarification of definitions for any terms relating to polymers,reference should be made to “Glossary of Basic Terms in Polymer Science”as published by the International Union of Pure and Applied Chemistry(“IUPAC”), Pure AppL Chem. 68, 2287-2311 (1996). However, any definitionexplicitly set forth herein should be regarded as controlling. Polymerscan be prepared from ethylenically unsaturated polymerizable monomersusing free radical polymerization or acid catalyzed polymerizationprocesses, or from appropriate condensation reactants (for example diolsand diisocyanates) using known condensation polymerization processes.

Unless otherwise indicated, the terms “polymer” and “polymeric” refer tohomopolymers and copolymers, each having a weight average molecularweight (M_(w)) of at least 5,000 as measured using gel permeationchromatography “(GPC).

The term “copolymer” refers to polymers that are derived from two ormore different monomers, in random order or a predetermined order (forexample, block or alternating order) along the polymer backbone. Thatis, each copolymer comprises at least two recurring units havingdifferent chemical structures.

The term “backbone” refers to the chain of atoms in a polymer to which aplurality of pendant groups can be attached. An example of such abackbone is an “all carbon” backbone obtained from the polymerization ofone or more ethylenically unsaturated polymerizable monomers. However,other backbones can include heteroatoms wherein the polymer is formed bya condensation reaction or some other means.

Many embodiments of the aqueous inkjet ink compositions used in thepresent invention, especially those used in continuous inkjet processesand inkjet printers generally have a viscosity of less than 3 centipose(less than 0.003 Pa-sec) at 25° C., or more likely less than 2centipoise (less than 0.002 Pa-sec), or even 1 centipoise or less (0.001Pa-sec or less), at 25° C., with a minimum viscosity being at least 0.8centipoise (at least 0.0008 Pa-sec) at the noted temperature. Viscositycan be measured at the noted temperature using a standard viscometer.

Aqueous Dispersions and Aqueous Inkjet Ink Compositions

The aqueous dispersions and aqueous inkjet ink compositions according tothe present invention are characterized as containing particles of oneor more pigment colorants that are present to provide a desiredcoloration upon application to a suitable substrate by ink jettingprocesses followed by drying or curing to remove the water and anyco-solvent(s). In most embodiments, the pigment colorants are notaqueous-dispersible and thus are prepared in aqueous dispersions usingone or more dispersing polymer(s), as described below. The pigmentcolorants can be either organic or inorganic in chemical nature.

In some embodiments, particles of titanium dioxide are used as thepigment colorant and are present in the aqueous inkjet ink compositionsin a sufficient amount to provide an ink jetted image having an opacityof at least 30%, or even at least 40%, for example in an amount of atleast 2 weight % and up to and including 25 weight %, or more typicallyof at least 4 weight % and up to and including 15 weight %, all based onthe total weight of the aqueous inkjet ink composition (including thewater weight). The titanium dioxide particles can be present with one ormore other type of pigment colorant particles (such as those describedbelow), or even with suitable dye colorants, but in most embodiments,the titanium dioxide particles are the only pigment colorant particlespresent in the aqueous dispersions and aqueous inkjet ink compositionsin the noted amounts. In most embodiments, the titanium dioxideparticles, or other pigment colorant particles described below used inthe aqueous dispersions and aqueous inkjet ink compositions, have a95^(th) percentile particle size of less than 200 nm, or even less than150 nm. Moreover, they generally also have a 50^(th) percentile particlesize of less than 130 nm, or typically of less than 100 nm. The titaniumdioxide or other pigment colorant pigments generally have a minimum or20^(th) percentile particle size of at least 50 nm, or even at least 75nm.

Other pigment colorants can be used to provide opaque ink jetted imagesalone or in addition to titanium dioxide (titania) as long as they havethe desired particle size characteristics (described above), are presentin the described amounts, to provide the desired opacity of at least 30%as defined above. For example, the aqueous dispersions and aqueousinkjet ink compositions can comprise particles of inorganic pigmentssuch as particles of zinc oxide, salts and esters of titanic acid,zirconium oxide (zirconia), aluminum phosphate, aluminum oxide(alumina), barium sulfate, calcium carbonate, magnesium oxide, silicondioxide (silica), and similar materials, as well as combinations of suchmaterials. Hollow sphere or other porous polymeric particles can also beused in combination with these inorganic pigments.

Other pigment colorants can also be used in aqueous dispersions andaqueous inkjet ink compositions, singly or in mixtures, in place oftitanium dioxide particles. Such pigment colorants can be used toprovide ink jetted images of various hues or colors other than the notedopaque (“white”) images. For example, useful aqueous dispersions andaqueous color inkjet ink compositions can be designed to provide cyan,magenta, yellow, or black images, all having the appropriate single ormixture of pigment colorants and dispersing polymers to provide thedesired image hue. Other useful aqueous dispersions and aqueous colorinkjet ink compositions can comprise pigment colorants to provideorange, red, green, brown, purple, or blue colored ink jetted images orany hue using combinations of such colors, and thus contain one or moresuitable pigment colorants to provide the desired ink jetted image hue.

A wide variety of organic and inorganic pigment colorants can be usedindividually or in combination. For example, a carbon black pigment canbe combined with a colored pigment such as a cyan copper phthalocyanineor a magenta quinacridine pigment. Useful pigment colorants aredescribed for example in U.S. Pat. No. 5,026,427 (Mitchell et al.), U.S.Pat. No. 5,141,556 (Matrick), U.S. Pat. No. 5,160,370 (Suga et al.), andU.S. Pat. No. 5,169,436 (Matrick), the disclosures of all of which areincorporated herein by reference.

Useful pigment colorants include but are not limited to, azo pigments,monoazo pigments, disazo pigments, azo pigment lakes, β-naphtholpigments, naphthol AS pigments, benzimidazolone pigments, disazocondensation pigments, metal complex pigments, isoindolinone andisoindoline pigments, quinacridone pigments, polycyclic pigments,phthalocyanine pigments, perylene and perinone pigments, thioindigopigments, anthrapyrimidone pigments, flavanthrone pigments, anthanthronepigments, dioxazine pigments, triarylcarbonium pigments, quinophthalonepigments, diketopyrrolo pyrrole pigments, iron oxide, and carbon blacks.Specific useful pigment colorants are also described in Col. 10 (lines66) to Col. 11 (line 40) of U.S. Pat. No. 8,455,570 (Lindstrom et al.),the disclosure of which is incorporated herein by reference.

The amount of one or more pigment colorants (for example, titaniumdioxide) in the aqueous dispersions can be any suitable amount that istypically used to make such compositions, for example at least 5 weight% and up to and including 80 weight %, based on the total weight of theaqueous dispersion (including the aqueous medium). The amount of thepigment colorants in the aqueous inkjet ink compositions is generallymuch less, such as at least 2 weight % and up to and including 25 weight%, or more typically at least 4 weight % and up to and including 15weight %, all based on the total weight of the aqueous inkjet inkcomposition (including the aqueous medium).

The aqueous inkjet ink compositions described herein generally alsocomprise one or more humectants that are generally water soluble orwater miscible organic solvents (sometimes known as “co-solvents”). Forexample, any water-soluble humectant or co-solvent known in the inkjetart that is compatible with the other components of the invention can beused. While an individual humectant or co-solvent can be employed,mixtures of two or more humectants or co-solvents, each of which impartsa useful property, can be used. Representative humectants andco-solvents useful in the aqueous inkjet ink compositions include butare not limited to the following compounds:

-   -   (1) alcohols, such as methyl alcohol, ethyl alcohol, n-propyl        alcohol, isopropyl alcohol, n-butyl alcohol, sec-butyl alcohol,        t-butyl alcohol, iso-butyl alcohol, furfuryl alcohol, and        tetrahydrofurfuryl alcohol;    -   (2) polyhydric alcohols (polyols), such as ethylene glycol,        diethylene glycol, triethylene glycol, tetraethylene glycol,        propylene glycol, dipropyleneglycol, the polyethylene glycols        with average molecular weights of at least 200 Daltons to and        including 5000 Daltons, the polypropylene glycols with average        molecular weights of at least 200 Daltons to and including 5000        Daltons, 1,2-propanediol, 1,3-propanediol, 1,2-butanediol,        1,3-butanediol, 1,4-butanediol, 1,2,4-butanetriol,        3-methyl-1,3-butanediol, 2-methyl-1,3-propanediol,        1,5-pentanediol, 1,6-hexanediol, 2-methyl-2,4-pentanediol,        1,7-heptanediol, 2-ethyl-1,3-hexane diol,        2,2,4-trimethyl-1,3-pentane diol, 1,8-octane diol, glycerol,        1,2,6-hexanetriol, 2-ethyl-2-hydroxymethyl-propanediol,        2-methyl-2-hydroxymethyl-propanediol, saccharides, sugar        alcohols, glycol, and thioglycol;    -   (3) polyoxygenated polyols and their derivatives such as        diglycerol, polyglycerols, glycerol ethoxides, glycerol        propoxides, glyceryths, alkylated and acetylated glyceryths,        pentaerythritol, pentaerythritol ethoxides, and pentaerythritol        propoxides and their alkylated and acetylated derivatives;    -   (4) nitrogen-containing compounds such as urea, 2-pyrrolidone,        N-methyl-2-pyrrolidone, imidazolidinone, N-hydroxyethyl        acetamide, N-hydroxyethyl-2-pyrrolidinone,        1-(hydroxyethyl)-1,3-imidazolidinone,        1,3-dimethyl-2-imidazolidinone, and        1,3-dihydroxy-2-imidazolidinone;    -   (5) sulfur-containing compounds such as 2,2′-thiodiethanol,        dimethyl sulfoxide and tetramethylene sulfone; and    -   (6) water soluble N-oxides such as 4-methylmorpholine-N-oxides.

Of these compounds, glycerol, ethylene glycol, and 2-pyrrolidinone (ormixtures thereof) are particularly useful. Useful polyhydric alcoholderivatives of glycerol include the glycerol ethoxides, glycerolpropoxides, and glyceryths. Useful humectants generally have meltingpoints below the typical operating temperature of the intended printersystem to avoid the formation of crystalline deposits on the print heador in the maintenance system. Practically, this means that the usefulhumectants have melting points below 30° C. or even below 20° C.

The one or more humectants (or co-solvents) can be present in an amountof at least 1 weight %, or at least 5 weight % and up to and including10 weight %, or up to and including 20 weight %, all based on the totalweight of the aqueous inkjet ink composition.

The particles of pigment colorants (such as particles of titaniumdioxide) are dispersed or kept suspended within the aqueous dispersionsand aqueous inkjet compositions described herein using one or moredispersing polymers as defined below.

The pigment colorants can be subjected to a milling or dispersingprocess to break up pigment particles into primary particles of thedesired size wherein a primary particle is defined as the smallestidentifiable subdivision in a particulate system.

For example, milling can be carried out using any type of grinding millsuch as a media mill, ball mill, a two-roller mill, a three-roller mill,a bead mill, or an air-jet mill, or by using an attritor or liquidinteraction chamber. During this milling process, the noted pigmentcolorants are generally suspended in a suitable aqueous medium that istypically the same or similar to the medium used to prepare the aqueousinkjet ink composition. Inert milling medium can be present in themilling step in order to facilitate breaking up the pigment colorantinto primary particles. Inert milling media can include such as glassballs, polymeric beads (such as crosslinked polystyrene beads),ceramics, metals, and plastics as described for example in U.S. Pat. No.5,891,231 (Gnerlich et al.).

Milling media are generally removed from the aqueous dispersion in asuitable manner before formulating the aqueous dispersion into anaqueous inkjet ink composition.

One or more dispersing polymers are generally used in the millingprocess in order to maintain primary particle stability and to preventsettling or agglomeration in the aqueous medium. Dispersing polymersuseful for this purpose are described below. Such materials can be addedprior to or during the milling step.

Pigment colorant particles (such as titanium dioxide particles) can bepresent in aqueous inkjet ink compositions in an amount of at least 2weight % and up to and including 25 weight %; a humectant can be presentthat has a molecular weight of less than 1,000; and the weight ratio ofthe pigment colorant particles to the dispersing polymer can be from19:1 to and including 2:1, from 13:1 to and including 4:1, or from 9:1to and including 5:1 (such weight ratios also apply to the aqueousdispersions). For example, the pigment colorant particles can betitanium dioxide particles and the aqueous inkjet ink composition iscapable of providing a dry image having an opacity of at least 30% asnoted above.

The presence of the dispersing polymer keeps the pigment colorantparticles (such as titanium dioxide particles) suitably dispersed sothat they have a 95^(th) percentile particle size of less than 200 nmand a 50^(th) percentile particle size of less than 130 nm.

The aqueous dispersions and aqueous inkjet ink compositions comprise oneor more dispersing polymers that are polymers comprising recurring unitsderived from ethylenically unsaturated polymerizable monomers(“monomers”) having a pendant group that is an carbocyclic aromaticgroup including but not limited to, a substituted or unsubstitutedphenyl group, a substituted or substituted benzyl group, a substitutedor unsubstituted phenoxy group, or a substituted or unsubstitutedphenethyl group. Examples of such monomers include but are not limitedto substituted or substituted styrene monomers, substituted orunsubstituted phenoxyethyl (meth)acrylate, benzyl (meth)acrylate, phenyl(meth)acrylate, and phenethyl (meth)acrylate. Particularly usefulmonomers of this type are substituted or unsubstituted styrene monomers.

The dispersing polymers are particularly designed by reacting asubstituted or unsubstituted styrene monomer to provide at least 25 mol% of recurring units from such monomers, as well as (meth)acrylic acid,maleic anhydride, or maleic acid, or combinations thereof to provide atleast 20 mol % of recurring units from such acidic monomers. Theseamounts are based on the total molar amount of recurring units in thedispersing polymer.

Such dispersing polymers also comprise recurring units derived frommethacrylic acid, and optionally additional recurring units derived fromother ethylenically unsaturated polymerizable monomers. The variousmonomers used to prepare these dispersing polymers can be obtained fromvarious commercial sources or prepared using known reactants andprocedures.

In such copolymers, the recurring units derived from a monomer having apendant carbocyclic aromatic group are present in an amount of at least20 mol %, or even at least 40 mol %, and up to and including 70 mol %,or up to and including 80 mol %, of the total recurring units in thecopolymer. It would be understood that the theoretical amount ofrecurring units can vary from the actual amount of recurring units.

Representative dispersing polymers of this type are described below inthe Examples.

The described dispersing polymers can have a molecular weight (M_(w)) asmeasured by GPC (using polystyrene standards) of at least 5,000 Daltonsor at least 6,000 Daltons, and up to and including 100,000 Daltons, orup to and including 50,000 Daltons.

Moreover, each of the noted dispersing polymers generally has an acidvalue of at least 100 and up to and including 500 or at least 150 and upto and including 300.

The aqueous inkjet ink compositions described herein generally comprisea suitable aqueous medium to “carry” the particles of one or morepigment colorants, one or more dispersing polymers, one or morehumectants, and any other components during formulation and use. Ingeneral, the aqueous medium is primarily water, meaning that watercomprises at least 50 weight %, or even at least 90 weight %, of all ofthe solvents in the aqueous inkjet ink composition. In addition, theaqueous medium generally comprises at least 70 weight % or at least 80weight % of the total weight of the aqueous inkjet ink composition. Whatis not water in the aqueous medium can be one or more water-soluble orwater-miscible organic solvents such as a glycol, amide, ether, urea,ester, or alcohol known in the art for this purpose. In mostembodiments, the aqueous medium is solely water.

The pH of each aqueous inkjet ink composition is generally at least 7and up to and including 11 or more likely at least 8 and up to andincluding 10. The pH can be obtained and adjusted by use of suitableacids or bases as is known in the art, such as an organic amine in asuitable amount. Buffers can be included to maintain the desired pH andsuch materials would be readily apparent to one skilled in the art,including those described in Cols. 17-19 of U.S. Pat. No. 8,455,570(noted above).

If desirable, one or more modified polysiloxanes can be present in theaqueous inkjet ink compositions. Examples of such materials areethoxylated or propoxylated silicone-based “surfactants” that can beobtained commercially under the trademarks SILWET® (CL Witco), and BYK®(Byk Chemie) such as BYK® 348 and 381, as well as Dow Corning DC67,DC57, DC28, DC500W, and DC51. One or more modified polysiloxanes can bepresent in a cumulative amount of up to and including 1 weight %, or atleast 2 weight % and up to and including 10 weight %, based on the totalweight of the aqueous inkjet ink composition.

Other additives that can be present in the aqueous inkjet inkcompositions, in amounts that would be readily apparent to one skilledin the art, include but are not limited to, polymer binders, thickeners,conductivity-enhancing agents, drying agents, waterfast agents,viscosity modifiers, pH buffers, antifoamants, wetting agents, corrosioninhibitors, biocides (such as Kordek and Proxel), fungicides, defoamers(such as SURFYNOL® DF110L, PC, MD-20, and DF-70 defoamers), non-siliconesurfactants (anionic, cationic, nonionic, or amphoteric) such asSURFYNOL® (Air Products) surfactants including

SURFYNOL″ 440 and 465 surfactants, UV radiation absorbers, antioxidants,and light stabilizers available under the trademarks TINUVIN® (Ciba) andIRGANOX® (Ciba), as well as other additives described in Col. 17 (lines11-36) of U.S. Pat. No. 8,455,570 (Lindstrom et al.). Examples of otheruseful non-silicone surfactants are provided in [0065]-[4006] of U.S.Patent Application Publication 2008/0207811 (noted above).

For example, in some embodiments of the invention, the aqueous inkjetink composition can include one or more polyurethanes in a total amountof at least 0.1 weight % and up to and including 8 weight %, of even atleast 1 weight % and up to and including 4 weight %, all based on thetotal weight of the aqueous inkjet ink composition.

Such polyurethanes are generally water-dispersible and can include butare not limited to polycarbonate polyurethanes, polyether polyurethanes,polyester polyurethanes, and polysiloxane polyurethanes, for example asdescribed in U.S. Pat. No. 8,430,492 (Falkner et al.) and U.S. PatentApplication Publication 2008/0207811 (Brust et al.), the disclosures ofwhich are incorporated herein by reference.

Ink Sets

The aqueous inkjet ink compositions designed for the present inventioncan be provided as part of an ink set containing multiple aqueous inkjetink compositions including an aqueous inkjet ink composition to providean opaque ink jetted image as well as one or more aqueous inkjet inkcompositions that provide a color other than “white.” Alternatively, twoor more aqueous inkjet ink compositions designed within the scope of thepresent invention can be combined to form suitable ink sets. Variouspigment colorants (including titanium dioxide) can be present in thevarious aqueous inkjet ink compositions combined in the ink sets. Theaqueous inkjet ink compositions in the ink set generally comprisedifferent pigment colorants, and they can comprise the same or differentaqueous soluble dispersing polymers.

Such ink sets can therefore include one or more aqueous inkjet inkcompositions, and at least one of the aqueous inkjet ink compositionscomprises at least one of the dispersing polymers described above andcan also include titanium dioxide as the colorant pigment, alone or incombination with another colorant pigment.

Unless otherwise specified above, the pigment colorants can be presentin the individual aqueous inkjet ink compositions in an amount of atleast 0.1 weight % and up to and including 30 weight %, or more likelyof at least 1 weight % and up to and including 15 weight %, or even atleast 1 weight % and up to and including 10 weight %, based on the totalweight of the individual aqueous inkjet ink composition.

Each of the aqueous inkjet ink compositions included in each ink set canhave a desirable pH of at least 7 and up to and including 11, or atleast 8 and up to and including 10 using suitable bases and buffersystems.

In addition, each aqueous inkjet ink composition included in an ink setcan have suitable viscosity of at least 1 centipose as measured at 25°C. using a suitable viscometer. In some embodiments designed for use incontinuous inkjet processes and inkjet printers, the individual aqueousinkjet ink composition has a viscosity of less than 3 centipoise (lessthan 0.003 Pa-sec) or even less than 2 centipose (less than 0.002Pa-sec) at 25° C.

Preparation of Aqueous Inkjet Compositions

Each aqueous inkjet ink composition described herein can be prepared bydispersing suitable pigment colorant particles (such as titanium dioxideparticles) in water (or aqueous medium) using suitable dispersingpolymers as described above, and mixing the resulting aqueous dispersionand other desired materials described above in suitable amounts.Specific details for how aqueous dispersions and aqueous inkjet inkcompositions can be prepared are provided below for the Examples.

Methods of Inkjet Printing

The aqueous inkjet ink compositions described above can be used to forma pigmented image on a suitable substrate (described below) for example,using inkjet printing. For example, the resulting inkjet printed imagecan be formed using any suitable inkjet application means (either DOD orCIJ inkjet printer apparatus) to provide colored or opaque printed areason the substrate.

For example, an opaque (“white”) image can be provided by inkjetprinting one or more aqueous inkjet ink compositions containing titaniumdioxide particles using thermal or piezoelectric drop-on-demand (DOD)printheads or continuous (CIJ) printheads that utilize electrostaticcharging devices and deflector plates or air deflection devices.Printing with each type of printhead and apparatus attached thereto canbe optimized using aqueous inkjet ink compositions designed withspecific properties, specific pigment colorant particles (such astitanium dioxide particles), and dispersing polymers as described abovein order to achieve reliable and accurate ink jetting and to provide thedesired opacity in the resulting inkjet printed image.

For example, a method for inkjet printing can comprise:

-   -   providing a substrate as described below, for example,        comprising a topmost layer comprising at least 30 weight % of        one or more soluble multivalent metal cations and at least 0.05        g/m² of a hydrophilic polymeric binder (as described below),    -   ink jetting an aqueous inkjet ink composition (as described        herein) from a main fluid supply as a continuous stream that is        broken into both printing drops and non-printing drops, forming        a pigmented printed image (such as an opaque “white” printed        image having an opacity of at least 30%) on the substrate with        the printing drops, and    -   collecting and returning the non-printing drops to the main        fluid supply.

In some embodiments of such methods, the aqueous inkjet ink compositionused in this particular method comprises particles of titanium dioxidein an amount of at least 4 weight % and up to and including 15 weight %,based on the total weight of the aqueous inkjet ink composition. Suchtitanium dioxide particles have the 95^(th) percentile particle size and50^(th) percentile particle size values described above, and aredispersed within the aqueous inkjet ink composition using a dispersingpolymer described above, such as a polymer comprising acidic groups, andthe weight ratio of the particles of titanium dioxide to the dispersingpolymer is as defined above.

In other embodiments of the present invention, the method of inkjetprinting comprises:

-   -   providing a suitable substrate (as described below), and    -   ink jetting an aqueous inkjet ink composition as described above        to form a printed image on the substrate,    -   wherein the aqueous inkjet ink composition comprises a pigment        colorant particles as described above (such as titanium dioxide        particles).

For example, in many embodiments, each aqueous inkjet ink compositionsprovided in an ink set has a dedicated delivery channel to avoid havingoppositely charged materials in the multiple inkjet ink compositionsinteract until they come into reactive association on the intendedsubstrate. The multiple aqueous inkjet ink compositions can be applied(ink jetted) either in the same pass or they can be applied inindependent multiple passes.

When the inkjet printing method is carried out using CU apparatus, themethod can include:

-   -   ink jetting the aqueous inkjet ink composition described above        from a main fluid supply as a continuous stream that is broken        into both printed spaced drops and non-printing drops; and    -   collecting and returning the non-printing drops to the main        fluid supply for use at a later time.

In some of such embodiments, the ink jetting can be carried out using adrop generator mechanism so that the printed spaced drops cover at least5% and up to and including 90% of a substrate area.

In addition, such an ink jetting method can include:

-   -   delivering a first aqueous inkjet ink composition as described        above to a first drop generator mechanism,    -   ejecting the first aqueous inkjet ink composition from the first        drop generator mechanism as first spaced drops while controlling        the first spaced drops to provide a first printed image on the        substrate;    -   delivering a second aqueous inkjet ink composition as described        above to a second drop generator mechanism, and    -   ejecting the second aqueous inkjet ink composition from a second        drop generator mechanism as second spaced drops while        controlling the second spaced drops.

An inkjet printed image can also be formed using a CIJ method,comprising:

-   -   delivering a first aqueous inkjet ink composition from a first        main fluid supply as a continuous stream that is broken into        both first spaced drops and first non-printing drops;    -   collecting and returning the first non-printing drops to the        first main fluid supply;    -   delivering a second aqueous inkjet ink composition from a second        main fluid supply as a continuous stream that is broken into        both second spaced drops and second non-printing drops; and    -   collecting and returning the second non-printing drops to the        second main fluid supply.

Each aqueous inkjet ink composition used in the inkjet printing methodscan be positioned in any one of the printhead ports intended for suchuse. Each aqueous inkjet ink composition can be positioned on anindividual carriage assembly, or all of the aqueous inkjet inkcompositions can be positioned on the same carriage assembly.

An inkjet printing method can be carried out to provide a suitableinkjet ink image on a suitable substrate (also known in the art as“receiver” or “receiver element”). Any individual substrate will havewhat would be understood as a total “printable” area onto which anaqueous inkjet ink composition can be ink jetted using suitableequipment and processes. The method can be used to apply an aqueousinkjet ink composition over a predetermined or areal percentage of thetotal printable area. Thus, the predetermined or areal percentage of thesubstrate to be covered by any individual aqueous inkjet ink compositioncan be as low as 0.5% and up to and including 100%, but more likely atleast 10% and up to and including 90%.

Suitable substrates can include but are not limited to, photoglossyreceivers or plain paper receivers such as bright white inkjet papersthat are commercially available from a number of commercial sources (asdescribed for example, in Col. 13, lines 28-34) of U.S. Pat. No.8,187,371 (noted above). The photoglossy receivers (also known asswellable media or microporous media) can be manufactured with a coatedlayer on an underlying paper support and are also useful for providingphotographic quality inkjet printed images. Some details of suchsubstrates are provided in Col. 13 (lines 37-51) of U.S. Pat. No.8,187,371 (noted above). Plain papers can be treated with multivalentsalts during or after manufacture as is well known in the art. Otheruseful substrates are described in U.S. Pat. No. 6,045,917 (Missell etal.), U.S. Pat. No. 5,605,750 (Romano et al.), U.S. Pat. No. 5,723,211(Romano et al.), and 5,789,070 (Shaw-Klein et al.).

Useful paper substrates include plain bond papers, surface-treatedpapers, coated or calendared business gloss papers, resin-coated papers,laminated substrates comprising both paper layers and polymeric filmlayers such as polyester film layers, and heavy stock papers. It is alsopossible to use fabrics, cardboard, plastic films (such as polyolefinsand polyester films), microporous materials, sheet metal, glass, and anyother substrate material known in the art. The substrate can betransparent, translucent, or opaque. For the “white” (opaque) aqueousinkjet ink compositions described for the present invention, transparentor translucent polymeric films are particularly useful.

The durability, opacity, and other properties of inkjet printed imagesprovided according to this invention can be improved by using treatedsubstrates that have been pretreated with a composition to enhance thequality of the inkjet printed images. This pretreatment is typicallydone prior to incorporation of the substrate into the inkjet printingapparatus (such as a continuous inkjet printing apparatus), but in someinstances, the substrate can be pretreated within the apparatus beforeapplication of the aqueous inkjet ink composition. One or both sides ofa substrate can be pretreated, or one side can be pretreated and theopposite surface left untreated.

For example, the substrate can be pretreated to form a “pretreatedreceiver element” by application of a coating (usually a colorlessmaterial), for example as described in U.S. Pat. No. 7,219,989 (Uerz etal.), the disclosure of which is incorporated herein by reference.

Alternatively, a substrate can be pretreated with a pretreatmentcomposition comprising a water-soluble multivalent metal ion salt, suchas but not limited to, a salt comprising one or more multivalent cationsincluding calcium, magnesium, barium, zinc, and aluminum cations, withcalcium and magnesium cations being particularly useful. Examples ofuseful multivalent metal cation salts include but are not limited to,calcium chloride, calcium acetate, calcium nitrate, magnesium chloride,magnesium acetate, magnesium nitrate, barium chloride, barium nitrate,zinc chloride, zinc nitrate, aluminum chloride, aluminumhydroxychloride, and aluminum nitrate. Other useful salts could bedetermined by a skilled artisan, and one or more of such multivalentmetal cation salts can be used in the pretreatment composition in anamount that would be readily apparent to one skilled in the art.

Such pretreatment compositions can also comprise a cationicpolyelectrolyte comprising amidine moieties, and the details of suchcompounds and their useful amounts are provided in U.S. Pat. No.8,562,126 (Xiang et al.), the disclosure of which is incorporated hereinby reference.

Besides applying with an inkjet printer apparatus (such as a continuousinkjet printer apparatus), the various aqueous inkjet ink compositionsdescribed herein can also be applied using other mechanical techniquesincluding but not limited to, rod coating, blade coating, gravurecoating, flexographic printing, extrusion hopper coating, curtaincoating, and spray coating.

A resulting printed substrate (or printed receiver element) can be anysuitable article, including but not limited to, documents, papercurrency, postage stamps, various packaging materials, fabrics,polymeric films or sheets, label for clothing, perfume and wine bottleslabels, lottery tickets, passports, drivers licenses, and other articlesthat would be readily apparent to one skilled in the art using theteaching provided herein.

It is also useful that inkjet printed drops of an aqueous inkjet inkcompositions be dried on the substrate after they have been inkjetprinted.

One sub-system common to most CIJ apparatus and methods and to some ofthe more recent DOD printing systems, is a recirculating fluid system,which constantly recirculates an aqueous inkjet ink composition eitherthrough or behind the nozzles used for jetting the aqueous inkjet inkcomposition. For the aqueous inkjet ink compositions described abovecontaining pigment colorant particles, the median particle size of suchparticles and the overall stability of the aqueous dispersion arecritical features due to the potentially abrasive nature of the aqueousdispersions. Larger particles or less stable aqueous dispersions aremore prone to cause premature wear or failure of the components of theinkjet printing system and fluid sub-system.

In some embodiments, an aqueous inkjet ink composition can be printed byemploying a plurality of drop volumes (or drop size) formed from thecontinuous fluid stream, with non-printing drops of a different volumethan printing drops being diverted by a drop deflection means into agutter for recirculation, as disclosed for example in U.S. Pat. No.6,588,888 (noted above), U.S. Pat. No. 6,554,410 (Jeanmaire et al.),U.S. Pat. No. 6,682,182 (Jeanmaire et al.), U.S. Pat. No. 6,793,328(Jeanmaire), U.S. Pat. No. 6,517,197 (Hawkins et al.), U.S. Pat. No.6,866,370 (Jeanmaire), and U.S. Pat. No. 6,575,566 (Jeanmaire et al.),U.S. Patent Application Publication 2003/0202054 (Jeanmaire et al.) thedisclosures of all of which are incorporated herein by reference.

In other embodiments, an aqueous inkjet ink composition can be printedonto at least part of an already applied image on a suitable substrateusing an apparatus capable of controlling the direction of the formedprinting and non-printing drops by asymmetric application of heat to thecontinuous stream of the aqueous inkjet ink composition that initializesdrop break-up and serves to steer the resultant drops, as disclosed forexample in U.S. Pat. No. 6,079,821 (Chwalek et al.) and U.S. Pat. No.6,505,921 (Chwalek et al.), the disclosures of both of which are hereinincorporated by reference. Useful agitation of the aqueous inkjet inkcomposition, heated main fluid supply, and inkjet print head andcomposition filtration means for CIJ ink compositions are described forexample in U.S. Pat. No. 6,817,705 (Crockett et al.), the disclosure ofwhich is incorporated herein by reference. Printer replenishing systemsfor maintaining quality of an aqueous inkjet ink composition and tocounter the effects of volatile component evaporation are described inU.S. Pat. No. 5,526,026 (Bowers) and U.S. Pat. No. 5,473, 350 (Mader etal.), the disclosures of which are incorporated herein by reference, andin EP 0 597 628 A1 (Loyd et al.).

It can be useful to regularly replenish the main fluid supply with theaqueous inkjet ink composition described herein to keep the reservoir atthe desired level during inkjet printing. Alternatively, water can beadded to the main fluid supply to compensate for evaporation duringinkjet printing. A skilled worker in the art would understand how toaccomplish these operations using the teaching provided in the art notedabove.

The present invention provides at least the following embodiments andcombinations thereof, but other combinations of features are consideredto be within the present invention as a skilled artisan would appreciatefrom the teaching of this disclosure:

1. A method of inkjet printing, comprising:

providing a substrate, and

ink jetting an aqueous inkjet ink composition to form a white printedimage on the substrate,

wherein the aqueous inkjet ink composition comprises particles oftitanium dioxide that are present in an amount of at least 4 weight %and up to and including 15 weight %, and the titanium dioxide particleshave a 95^(th) percentile particle size of less than 200 nm and a 50thpercentile particle size of less than 130 nm, and

the titanium dioxide particles are dispersed within the aqueous inkjetink composition with a dispersing polymer that is astyrene-(meth)acrylic acid polymer, styrene-maleic anhydride polymer, orstyrene-maleic acid polymer, wherein such dispersing polymer comprisesat least 25 mol % of styrene recurring units and at least 20 mol % ofrecurring units derived from (meth)acrylic acid, maleic anhydride ormaleic acid, respectively; and the weight ratio of the titanium dioxideparticles to the dispersing polymer is from 19:1 to and including 2:1.

2. A method of inkjet printing, comprising:

providing a substrate,

ink jetting an aqueous inkjet ink composition from a main fluid supplyas a continuous stream that is broken into both printing drops andnon-printing drops, forming a white printed image on the substrate withthe printing drops, and

collecting and returning the non-printing drops to the main fluidsupply,

wherein:

the aqueous inkjet ink composition comprises particles of titaniumdioxide that are present in an amount of at least 4 weight % and up toand including 15 weight %, and the titanium dioxide particles have a95^(th) percentile particle size of less than 200 nm and a 50^(th)percentile particle size of less than 130 nm, and

the titanium dioxide particles are dispersed within the aqueous inkjetink composition with a dispersing polymer that is astyrene-(meth)acrylic acid polymer, styrene-maleic anhydride polymer, orstyrene-maleic acid polymer, wherein such dispersing polymer comprisesat least 25 mol % of styrene recurring units and at least 20 mol % ofrecurring units derived from (meth)acrylic acid, maleic anhydride ormaleic acid, respectively; and the weight ratio of the titanium dioxideparticles to the dispersing polymer is from 19:1 to and including 2:1.

3. The method of embodiment 1 or 2, wherein the weight ratio of thetitanium dioxide particles to the dispersing polymer is from 9:1 to andincluding 5:1.

4. The method of any of embodiments 1 to 3, wherein the dispersingpolymer has a weight average molecular weight of at least 5,000 and upto and including 100,000, and an acid value of at least 100 and up toand including 500.

5. The method of any of embodiments 1 to 4, wherein the titanium dioxideparticles have a 95^(th) percentile particle size of less than 200 nmand a 50th percentile particle size of less than 130 nm.

6. The method of any of embodiments 1 to 5, wherein:

the titanium dioxide particles that have a 95^(th) percentile particlesize of less than 200 nm and a 50^(th) percentile particle size of lessthan 130 nm;

the dispersing polymer has a weight average molecular weight of at least5,000 and up to and including 100,000, and an acid value of at least 100and up to and including 500; and

the weight ratio of the titanium dioxide particles to the dispersingpolymer is from 9:1 to and including 5:1.

7. The method of any of embodiments 1 to 6, wherein the aqueous inkjetink composition further comprises a polyurethane in an amount of atleast 0.1 weight % and up to and including 8 weight %, based on thetotal weight of the aqueous inkjet ink composition. 8. The method of anyof embodiments 1 to 7, wherein the polyurethane is a polycarbonatepolyurethane that is present in an amount of a least 1 weight % and upto and including 4 weight %, based on the total weight of the aqueousinkjet ink composition.

The following materials were prepared for use in the Examples that areprovided to illustrate the practice of this invention and are not meantto be limiting in any manner.

Synthesis of Dispersing Polymer, Poly(styrene-co-methacrylic acid)(59:41 molar ratio):

Styrene (50.0 g, 0.480 mol) and methacrylic acid (28.2 g, 0.328 mol)were placed in a 500 ml flask and tetrahydrofuran (182.0 g) was addedfor 30% solids. 2,2′-Azobis(2-methylbutyronitrile) (AMBN) (1.56 g,0.0081 mol) was added and the solution was purged with nitrogen for 60minutes and placed in a preheated oil bath at 70° C. for 17 hours. Thesolution was cooled and precipitated into toluene. The precipitate wasfiltered, dried, re-dissolved in tetrahydrofuran at 25% solids,precipitated into toluene, filtered, and dried to obtain 41.4 g (53%yield) of poly(styrene-co-methacrylic acid).

General Aqueous Dispersion Preparation:

Aqueous dispersions containing titanium dioxide as the sole pigmentcolorant were prepared according to the present invention using thefollowing materials:

Titanium dioxide (Kronos 1000), 12.5 g;

Dispersing polymer, 1.9 g;

Dimethylethanol amine, 0.6 g; and

Water, 85 g.

In a dispersion vessel, the dispersing polymer and dimethylethanol aminewere mixed together to neutralize the dispersing polymer, followed byaddition of the titanium dioxide and water. SEPR fused zirconia silicagrinding media (250 ml, 0.6-0.8 mm diameter beads) were added to themixture, the vessel was sealed and placed on a roller mill (about 97rpm), and the mixture was milled for 13 days to provide an aqueoustitanium dioxide dispersion.

Preparation of Aqueous Inkjet Ink Compositions with TiOz as Sole PigmentColorant:

To prepare aqueous inkjet ink compositions according to the presentinvention for use in continuous inkjet printing (CU) methods, thefollowing materials were mixed together in a glass jar:

Aqueous TiO2 dispersion (described above), 69 g;

Surfynol® 440 surfactant, 0.06 g;

Glycerol humectant, 4.5 g;

Surfynol'' DF-110L (20% active), 0.08 g; and

Water, 26.4 g.

To prepare aqueous inkjet ink compositions according to the presentinvention for using in drop on demand printing (DOD) methods, thefollowing materials were mixed together in a glass jar:

Aqueous TiO2 dispersion (described above), 17.25 g;

Surfynol® 465 surfactant, 0.25 g;

Glycerol humectant, 2 g;

Diethylene glycol humectant, 1 g;

1,5-Pentanediol, 1 g; and

Water, 3.5 g.

Each inkjet ink composition was evaluated using coatings onto asubstrate. Each inkjet ink composition was coated onto the substrateusing a hopper at 0.66 cm³/ft² (7.13 cm³/m²) wet coverage, followed bydrying at room temperature.

The substrate used in the evaluations comprised poly(ethyleneterephthalate) film samples (0.004 inch or 0.01 cm thickness) that hadbeen coated with poly(vinyl alcohol) (PVA) (GH23 from Nippon Goshei) toprovide a dried subbing coating of 50 mg/ft² (525 mg/m²) of PVA(Substrate A).

Particle Size:

Colorant (titanium dioxide) particle size was measured using a Nanotrac150 ultrafine particle analyzer available from Microtrac.

Opacity:

The opacity of coatings of aqueous inkjet ink compositions was measuredusing an X-rite densitometer and calculations as described above.

Potential to Settle:

A sample (2 g) of an aqueous inkjet ink composition was diluted to 20 gwith distilled water to provide Dilution A. A sample (1 g) of Dilution Awas placed into a 100 ml volumetric flask and further diluted to 100 mlwith distilled water. Using a UV-VIS spectrophotometer, the absorbance(Ainitial) was measured at 500 nm. A sample (10 ml) of Dilution A wasplaced into a 10 ml graduated cylinder, covered with parafilm, and heldat room temperature for 1 week, after which 2 ml of the sample wasremoved from the top of the graduated cylinder using a pipet. A sample(1 g) of this removed dispersion was placed into a 100 ml volumetricflask and diluted to 100 ml with distilled water and the absorbance wasthen measured at 500 nm (A_(1 wk)). The % residual absorbance wascalculated as% Residual Absorbance=100*A _(1wk) /A _(initial)The lower the percentage the greater the potential to settle (lessstable aqueous inkjet ink composition).Incubation Stability:

A sample of the aqueous inkjet ink composition was placed into a sealedvial and incubated at 60° C. for 4 weeks. The particle size of thecolorant was measured before and after this incubation period. Anygrowth in particle size indicates instability of the aqueous inkjet inkcomposition.

INVENTION EXAMPLE 1 Aqueous Inkjet Ink Composition

An aqueous dispersion was prepared according to the present inventionwith titanium dioxide using poly(styrene-co-methacrylic acid (59:41 molratio) (Dispersant B) as the dispersing polymer at 15 weight % based onthe weight of the titanium dioxide.

Two aqueous inkjet ink compositions were prepared using this aqueousdispersion as described above for inkjet printing using DOD or CIJinkjet printing apparatus. The DOD apparatus was a Kodak 5300 inkjetprinter and the inkjet ink composition was successfully inkjet printedonto Substrate A to provide a “white” image with 69% opacity. CIJ inkjetprinting was carried out using a CIJ apparatus and the inkjet inkcomposition was successfully inkjet printed onto Substrate A to providea “white” image with 55% opacity.

Sample A of the aqueous dispersion containing Dispersant B describedabove was milled for 13 days and Sample B of the same aqueous dispersionwas milled for 4 hours. Inkjet ink compositions were then prepared fromthese milled samples using the formulation described above, and testedfor the “potential to settle” of the titanium dioxide particles usingthe evaluation described above. The results in the following TABLE Ishow that when the aqueous dispersion was not sufficiently milled, thetitanium dioxide particle size was too large and agglomeration(instability) occurred after 1 week. These results thus show thecriticality of having the desired particle size distribution for usingtitanium dioxide to provide desired dispersibility and the formation ofa dry “white” image with desired opacity.

TABLE I 50% 95% Potential to Aqueous Milling Particle Particle Settle: %residual Dispersion Time Size (nm) Size (nm) absorbance Sample A 13 days77 115 98 Sample B  4 hours 227 322 20

COMPARISON EXAMPLE 1 and INVENTIONS EXAMPLES 2-6

Aqueous dispersions and aqueous inkjet ink compositions containingtitanium dioxide and samples of polystyrene-acrylic acid) as dispersingpolymers were prepared as described above and applied to a substrate,and the evaluation of results are shown in the following TABLES II andIII.

These results show that when the appropriate acidic styrene dispersingpolymers were used according to the present invention, the titaniumdioxide particle size was kept low to provide stability of the aqueousinkjet ink composition and the resulting applied coating showed desiredopacity without the use of a substrate that was treated with multivalentmetal cations.

TABLE II Ink TiO₂ Dispersant Particles Sizes Incubation StabilityPotential to Polymer Styrene 50% 95% 50% 95% Settle to MethacrylicPercentile Percentile Percentile Percentile % Opacity % Residual AcidMol Ratio Example (nm) (nm) (nm) (nm) On Substrate A Abs 75:25 Invention2 123 176 120 172 50 86 50:50 Invention 3 128 191 122 178 60 100 50:50Invention 4 118 172 107 193 72 90 25:75 Invention 5 122 187 115 179 73100 59:41 Invention 6 86 130  82 134 55 90 E Comparison 1 112 204 NA NA8 85 NA = not available Dispersion E was poly(benzylmethacrylate-co-methacrylic acid) (50:50 mol ratio);

TABLE III Ink TiO₂ Dispersant Particles Sizes Incubation StabilityPotential to Cray Valley Mol Ratio of 50% 95% 50% 95% Settle AnionicStyrene to Percentile Percentile Percentile Percentile % Opacity %Residual Polymers* Methacrylic Acid (nm) (nm) (nm) (nm) On Substrate AAbs SMA 2000 2:1 74 104 116 188 40 84 SMA EF-30 3:1 73 120 67 101 32 104SMA EF-40 4:1 87 137 83 124 52 100 SMA EF-60 6:1 96 154 98 148 63 82*Cray Valley polymers available from Cray Valley

Aqueous dispersions and aqueous inkjet ink compositions containingvarious commercial samples of titanium dioxide and apoly(styrene-methacrylic acid) (59:41 mol ratio) as a dispersing polymerwere prepared as described above and coated onto Substrate A, and theevaluation of results are shown in the following TABLE IV.

These results show that when the appropriate acidic styrene dispersingpolymers were used as dispersing polymers according to the presentinvention, the size of particles from various types of titanium dioxidewere kept low to provide stability of the aqueous inkjet inkcompositions. The resulting coatings exhibited desired opacity withoutthe use of a substrate that has been treated with multivalent metalcations. The results also show that different pigment colorant todispersing polymer ratios provide aqueous dispersions that are stable inthe aqueous inkjet ink compositions and provide the desired opacity whendried on the substrate.

TABLE IV Ink TiO₂ Particles Potential TiO₂ to Sizes Incubation Stabilityto Settle Dispersing 50% 95% 50% 95% % Titanium Polymer PercentilePercentile Percentile Percentile % Opacity Residual Dioxide Type(Supplier) Ratio (nm) (nm) (nm) (nm) On Substrate A Abs Kronos 1000Anatase (Kronos) 6.6:1 82 109 71 105 45 107 TiPure R104 Rutile (DuPont)6.6:1 110 177 104 186 75 87 Kronos 2310 Rutile (Kronos) 6.6:1 84 138 67101 48 97 Kronos 1000 Anatase (Kronos) 13.3:1  86 144 86 122 41 98Kronos 1000 Anatase (Kronos)  10:1 69 106 61 92 45 100

INVENTION EXAMPLE 7

Aqueous dispersions and aqueous inkjet ink compositions containingtitanium dioxide and samples of poly(styrene-acrylic acid) as dispersingpolymers were prepared as described above for the CU printing method. Tothese were added at 2 weight % with respect to the aqueous inkjet inkcomposition, either a polyether-polyurethane (PEPU, made fromisophorone, bishydroxymethylpropionic acid, and a polycarbonate dial) ora polycarbonate-polyurethane (PCPU, made from isophorone,bishydroxymethylpropionic acid, butanediol, and a polytetramethyleneglycol). Each aqueous inkjet ink composition was applied to Substrate Aand the evaluation of results are shown in the following TABLE V.

These results show that the incorporation of a polyurethane with thepoly(styrene-acrylic acid) dispersing polymer provided stable aqueousinkjet ink compositions and improved opacity without the use of asubstrate that has been treated with multivalent metal cations.

TABLE V Ink TiO₂ Dispersant Particles Sizes Incubation StabilityPotential to Polymer:Styrene 50% 95% 50% 95% Settle to MethacrylicPercentile Percentile Percentile Percentile % Opacity % Residual AcidMol Ratio Polyurethane (nm) (nm) (nm) (nm) On Substrate A Abs 59:41 None79 118 72  98 56 100 59:41 2% PEPU 83 118 NA NA 67 98 59:41 2% PCPU 79118 89 120 74 100 NA = not available

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

The invention claimed is:
 1. A method of inkjet printing, comprising:providing a substrate, and ink jetting an aqueous inkjet ink compositionto form a white printed image on the substrate, the white printed imagehaving an opacity of at least 30%, wherein the aqueous inkjet inkcomposition comprises particles of titanium dioxide that are present inan amount of at least 4 weight % and up to and including 15 weight %,and the titanium dioxide particles have a 95^(th) percentile particlesize of less than 200 nm and a 50^(th) percentile particle size of lessthan 130 nm, and the titanium dioxide particles are dispersed within theaqueous inkjet ink composition with a dispersing polymer that is astyrene-(meth)acrylic acid polymer, styrene-maleic anhydride polymer, orstyrene-maleic acid polymer, wherein such dispersing polymer comprisesat least 25 mol % of styrene recurring units and at least 20 mol % ofrecurring units derived from (meth)acrylic acid, maleic anhydride ormaleic acid, respectively; and the weight ratio of the titanium dioxideparticles to the dispersing polymer is from 19:1 to and including 2:1.2. The method of claim 1, wherein the weight ratio of the titaniumdioxide particles to the dispersing polymer is from 9:1 to and including5:1.
 3. The method of claim 1, wherein the dispersing polymer has aweight average molecular weight of at least 5,000 and up to andincluding 100,000, and an acid value of at least 100 and up to andincluding
 500. 4. The method of claim 1, wherein the titanium dioxideparticles have a 95^(th) percentile particle size of less than 200 nmand a 50^(th) percentile particle size of less than 130 nm.
 5. Themethod of claim 1, wherein: the titanium dioxide particles that have a95^(th) percentile particle size of less than 200 nm and a 50^(th)percentile particle size of less than 130 nm; the dispersing polymer hasa weight average molecular weight of at least 5,000 and up to andincluding 100,000, and an acid value of at least 100 and up to andincluding 500; and the weight ratio of the titanium dioxide particles tothe dispersing polymer is from 9:1 to and including 5:1.
 6. The methodof claim 1, wherein the aqueous inkjet ink composition further comprisesa polyurethane in an amount of at least 0.1 weight % and up to andincluding 8 weight %, based on the total weight of the aqueous inkjetink composition.
 7. The method of claim 6, wherein the polyurethane is apolycarbonate polyurethane that is present in an amount of a least 1weight % and up to and including 4 weight %, based on the total weightof the aqueous inkjet ink composition.
 8. A method of inkjet printing,comprising: providing a substrate, ink jetting an aqueous inkjet inkcomposition from a main fluid supply as a continuous stream that isbroken into both printing drops and non-printing drops, forming a whiteprinted image on the substrate with the printing drops, the whiteprinted image having an opacity of at least 30%, and collecting andreturning the non-printing drops to the main fluid supply, wherein: theaqueous inkjet ink composition comprises particles of titanium dioxidethat are present in an amount of at least 4 weight % and up to andincluding 15 weight %, and the titanium dioxide particles have a 95^(th)percentile particle size of less than 200 nm and a 50^(th) percentileparticle size of less than 130 nm, and the titanium dioxide particlesare dispersed within the aqueous inkjet ink composition with adispersing polymer that is a styrene-(meth)acrylic acid polymer,styrene-maleic anhydride polymer, or styrene-maleic acid polymer,wherein such dispersing polymer comprises at least 25 mol % of styrenerecurring units and at least 20 mol % of recurring units derived from(meth)acrylic acid, maleic anhydride or maleic acid, respectively; andtheweight ratio of the titanium dioxide particles to the dispersingpolymer is from 19:1 to and including 2:1.
 9. The method of claim 8,wherein the weight ratio of the titanium dioxide particles to thedispersing polymer is from 9:1 to and including 5:1.
 10. The method ofclaim 8, wherein the dispersing polymer has a weight average molecularweight of at least 5,000 and up to and including 100,000, and an acidvalue of at least 100 and up to and including
 500. 11. The method ofclaim 8, wherein the titanium dioxide particles have a 95^(th)percentile particle size of less than 200 nm and a 50^(th) percentileparticle size of less than 130 nm.
 12. The method of claim 8, wherein:the titanium dioxide particles that have a 95^(th) percentile particlesize of less than 200 nm and a 50^(th) percentile particle size of lessthan 130 nm; the dispersing polymer has a weight average molecularweight of at least 5,000 and up to and including 100,000, and an acidvalue of at least 100 and up to and including 500; and the weight ratioof the titanium dioxide particles to the dispersing polymer is from 9:1to and including 5:1.
 13. The method of claim 8, wherein the aqueousinkjet ink composition further comprises a polyurethane in an amount ofat least 0.1 weight % and up to and including 8 weight %, based on thetotal weight of the aqueous inkjet ink composition.
 14. The method ofclaim 13, wherein the polyurethane is a polycarbonate polyurethane thatis present in an amount of a least 1 weight % and up to and including 4weight %, based on the total weight of the aqueous inkjet inkcomposition.